*{
    margin:0px;
    padding:0px;
    text-align: center;
    font-size: 48px;
}
#head,#foot,#main{
    height:100px;
    width: 1200px;
    background-color: burlywood;
    line-height:100px;
    margin:0 auto;
    min-width:320px;
}

#head ul{width:80%;}
#head ul li{
    float: left;
    width: 20%;
    text-align: center;
    list-style: none;
    font-size: 20px;
}
#head div{
    display: none;
    font-size: 20px;
    height: 30px;
    width: 100px;
    background-color: yellow;
    float:right;
    margin-top: 35px;
    line-height: 30px;
}
#main{
    height: auto;
    margin: 10px auto;
    overflow: hidden;
}
.left,.center,.right{
    height: 600px;
    line-height: 600px;
    float: left;
    width: 20%;
    background-color: yellowgreen;
}
.center{
    width: 60%;
    border-left: 10px solid #FFFFFF;
    border-right: 10px solid #FFFFFF;
    box-sizing: border-box;
}
@media only screcn and (max-width: 1200px){/*当屏幕宽度大于1200px时会执行的CSS代码*/
    #head,
    #foot,
    #main{
        width: 100%;
    }
}
@media only screen and (max-width:980px){/*当屏幕宽度小于980px时会执行的CSS代码*/
    .right {display: none;}
    .left{ width: 30%;}
    .center{
        width:70%;
        border-right: hidden;
    }
}
    @media only screen and (max-width:640px){/*当屏幕宽度小于640px时会执行的CSS代码*/
    .left,.center,.right{
        width: 100%;
        height: 200px;
        line-height: 200px;
        display: block;
    }
    .center{
        border: hidden;
        border-top: 10px solid #FFFFFF;
        border-bottom: 10px solid #FFFFFF;
        height: 600px;
        line-height:600px;
    }
    #head ul{display: none;}
    #head div{display: block; }
}